Electrostimulation can be delivered to a heart, such as to trigger or to spatially coordinate a responsive cardiac depolarization and accompanying heart contraction. An implantable or other ambulatory cardiac function management device, such as a pacer, a cardioverter, a defibrillator, a cardiac contractility modulation (CCM) or a cardiac resynchronization therapy (CRT) device can be configured to include capability for monitoring cardiovascular function or for generating or providing such stimulation to the heart, such as for triggering or spatially coordinating responsive heart contractions. Such electrostimulations can be delivered via two or more electrodes. For example, such electrodes can include one or more electrodes that can be located at or near the distal end of one or more implantable leadwires, which can be connected to an implantable cardiac function management device. Such electrodes can also include one or more electrodes located at the implantable cardiac function management device.
Electrostimulation can be either cathodal or anodal, or a combination of anodal and cathodal. In cathodal stimulation, a more negative electrode (e.g., in an arrangement of electrodes) “captures” contractile cardiac tissue to trigger the resulting cardiac depolarization and accompanying heart contraction. In anodal stimulation, a more positive electrode (e.g., in an arrangement of electrodes) triggers the resulting cardiac depolarization and accompanying heart contraction. Various factors can influence whether anodal or cathodal stimulation occurs. For example, relative electrode size of an anode vs. a cathode can determine whether anodal, cathodal, or a combination of anodal and cathodal stimulation occurs. An electrode having a smaller surface area can have a greater current density through nearby tissue than a larger electrode. Greater current density can lower the threshold amount of energy needed to evoke a resulting cardiac depolarization and accompanying heart contraction.
The threshold amount of energy needed to evoke a resulting cardiac depolarization and accompanying heart contraction can differ for anodal stimulation vs. cathodal stimulation. Because cathodal stimulation typically needs less energy to accomplish the desired result of capturing cardiac tissue to evoke a resulting heart contraction, it can be preferred over anodal stimulation. This is because the useful life of an implantable device can depend on how quickly its battery is depleted. If a desired stimulation can be accomplished using less energy, that can prolong the useful life of an implanted device before the device is explanted. Such explantation can expose a subject to increased health care cost and the potential risk of infection that is associated with any invasive surgical procedure. Because of these and other potential differences in anodal vs. cathodal stimulation, it can be useful to detect whether a particular stimulation is anodal in nature. A description of anodal capture, cathodal capture, and left ventricular cardiac excitation is provided in J. Paul Mounsey and Stephen B. Knisley, “ANODAL CAPTURE, CATHODAL CAPTURE, AND LEFT VENTRICULAR CARDIAC EXCITATION,” Journal of Cardiovascular Electrophysiology, Vol. 20, No. 6, June 2009, pages 650-652, which is incorporated herein by reference in its entirety.
In Lu U.S. Pat. No. 6,687,545, anodal stimulation is detected by the absence of a delay between a bipolar stimulation pulse and an evoked response sensed at an electrode functioning as the anode during stimulation.
In Bjorling WO 2008/130293, anodal stimulation is detected using a paced depolarization integral (PDI) initialization test. For a 0.5 ms pacing pulsewidth, pacing amplitude is varied, and a portion of the evoked cardiac electrogram is integrated to provide a PDI. For cathodal stimulation, PDI vs. amplitude exhibits two distinct plateaus, by contrast, for anodal stimulation, three distinct plateaus are exhibited.
Also in Bjorling WO 2008/130293, anodal stimulation is detected by measuring a temporal distance between an applied stimulation pulse and a morphological feature—the morphological feature being the minimum value of the evoked response signal. Bjorling WO 2008/130293 notes that the temporal distance is shorter for anodal capture than cathodal capture, when the intracardiac electrogram (IEGM) is measured between a left ventricular (LV) ring electrode and a case electrode at the case housing the electronics of the implantable device, and that the temporal distance is longer for anodal capture than cathodal capture when the IEGM is measured between a LV tip electrode and the case electrode.